It has been 50 years since the only prisoners to escape the US Penitentiary Alcatraz in San Francisco pulled off one of the most legendary unsolved crimes in American history. Still 50 years later the US Marshals Service says it remains "diligent in the manhunt for Frank Morris and brothers Clarence and John Anglin as they are the only men to escape from Alcatraz Island in San Francisco who remain unaccounted for."

"No matter where the leads take us, or how many man hours are spent on this historic case, the Marshals Service will continue to investigate to the fullest extent possible," said David Harlow, assistant director, U.S. Marshals Investigative Operations Division in a statement. To back up the notion, the Marshals said they will continue to pursue the escapees until they are either arrested, positively determined to be deceased or reach the age of 99. Morris, who was in prison for burglary of a bank would be 85 years old, Clarence Anglin who was in prison for armed robbery would be 81 and John Anglin was in Alcatraz for armed robbery as well, would be 82 if they managed to survive the escape, which has never been proven one way or the other.

According to the Marshals, on the night of June 11, 1962, the three escaped through vents in the prison and made their way to the northeast part of the island, where they inflated the makeshift raft made from more than 50 raincoats and put on three life preservers and slipped into the water. Varied reports stated that the inmates either drowned or made their escape via nearby Angel Island. A fourth inmate, Allen West, was involved in planning the escape, but he never made it out of his prison cell. The known details of the escape were provided by West during several interviews, the Marshals stated.

The elaborate escape plan was in the works for more than a year and aside from the life raft and life preservers, included the making of lifelike dummies to ruse guards on night bed checks and enlarged ventilation holes in their cell walls, which they used spoons to create and concealed with cardboard replicas of vent covers, the Marshals said.

The Marshals Service adopted the case from the FBI in 1979. Since that time, countless deputy U.S. marshals have worked the case and investigated thousands of leads in almost every state in the country and a few foreign countries. They used media venues such as the TV show America's Most Wanted to generate tips and additional investigative information. The possibility of survival steered investigators to unusual and detailed leads to suspected whereabouts of the escapees. One example occurred in 2010, when an unmarked grave, claimed to be that of an escapee, was exhumed but failed to offer positive identification.

In a 1993 interview with that program, US Marshals Service Acting Director John Twomey said, "We know they were young and vigorous, that they had the physical ability to survive and that they had a well-thought-out scheme."

The FBI wrote about the case in 2007 stating: Plenty of people have gone to great lengths to prove that the men could have survived, but the question remains: Did they? Our investigation at the time concluded otherwise, for the following reasons:

Crossing the Bay. Yes, youngsters have made the more than mile-long swim from Alcatraz to Angel Island. But with the strong currents and frigid Bay water, the odds were clearly against these men.

Three if by land. The plan, according to our prison informant, was to steal clothes and a car once on land. But we never uncovered any thefts like this despite the high-profile nature of the case.

Family ties. If the escapees had help, we couldn't substantiate it. The families appeared unlikely to even have the financial means to provide any real support.

Missing in action. For the 17 years we worked on the case, no credible evidence emerged to suggest the men were still alive, either in the U.S. or overseas.

Some interesting facts about Alcatraz:

During its 29 years of operation, the penitentiary claimed no prisoner had successfully escaped. A total of 36 prisoners made 14 escape attempts, two men trying twice; 23 were caught, six were shot and killed during their escape, two drowned, and five are listed as "missing and presumed drowned".

Among its myriad infamous prisoners, Alcatraz held Al Capone, George "Machine Gun" Kelly, Alvin "Creepy Karpis" Karpowicz and a man who has been in the news lately, James "Whitey" Bulger.